Method and apparatus for operating a single sensor to have the properties of an array of sensors

ABSTRACT

Seismic signals in the area of detection associated with a single geophone sensor are received by the sensor, amplified in a variable gain amplifier, and analyzed by a processor to determine whether they are produced by an intruder moving in the detection area. The output of the processor is applied to logic circuitry which automatically decreases the amplifier gain, and thus the size of the detection area, when the received signal is produced by an intruder. If the processor produces outputs indicating intrusion of the detection area for a prescribed number of successively smaller areas of detection within a predetermined time interval, logic circuitry produces an alarm signal indicating that an intruder is in a detection area.

United States Patent Lucas 1 Nov. 14, 1972 METHOD AND APPARATUS FOROPERATING A SINGLE SENSOR TO Primary Examiner-John W. Caldwell HAVE THEPROPERTIES OF AN Assistant Examiner-Michael Slobasky ARRAY OF SENSORSAnomeyNorman J. OMalley, Russell A. Cannon 72 Inventor: Raymond DavidLucas, Sunnyvale, and Lawler Calif [57] ABSTRACT [73] Asslgnee: GTESylvan Incorporated Seismic signals in the area of detection associatedwith Filed! March 1970 a single geophone sensor are received by thesensor, [21] AppL No: 16,035 amplified in a variable gain amplifier, andanalyzed by a processor to determine whether they are produced by anintruder moving in the detection area. The out- [52] US. Cl ..340/258 R,340/261, 340/15, put f h processor i applied to logic circuitry which343/73 automatically decreases the amplifier gain, and thus [5 1] Int.Cl, ..G08b 13/00 the Size of the detection area when the received Signal[58] Field surchm34olzss 258 25 is produced by an intruder. If theprocessor produces 340/15, 15.5, 258 D, 276, 343/73 outputs indicatingintrusion of the detection area for a prescribed number of successivelysmaller areas of de- [56] Reerences Cited tection within a predeterminedtime interval, logic cir- UNITED STATES PATENTS cuiriry producgs analarm signal indicating that an intru er IS in a etectlon area.3,599,197 8/197] Boyko ..340/258 R 3,036,219 5/1962 Thompson ..340/258 BUX 17 Claims, 6 Drawing Figures T I 3 I? I9 30" 30' 30 23 24 7 K l wasJIM/C f I figs oq .vgznsau ESSA- AND 5:23, TRANSMITTER AMPLIFIER a 4|[:12 a I if 26 25 I0 I2 I f 2| 45 1 l 33 m 22 m l I AND 32 TIM ER n I 38C I l 5 AND 37 o I FLIP- 5 F FLOP ats LIP- FLOP 6 i o RESET 1 39 RESET43 l l I l I I l g R g l PATENTED B 141973 3. 702,998

sum 2 0r 3.

CONTROL SIGNAL CIRCUIT PROCESSOR I 77 78 ,w 2a A [28 B SENSOR CONTROLSIGNAL AND ALARM cmcun' CIRCUIT PROCESSOR DEVICE CONTROL SIGNAL CIRCUITPROCESSOR 29 A 29 B INVENTOR.

RAYMOND D. LUCAS GYM AGENT W i i 7 PATENTEDIIIII I4 l972 3,702,998

SHEEI 3 BF 3 SENSOR V PREAMP TRANSMITTER I RECEIVER PROCESS'NG CIRCUITCIRCUIT MIXER IF LIMITER- OSCILLATOR v AMPLIFIER DISCRIMINATOR LOCAL 62PROCESSING OSCILLATOR CIRCUIT (3 (l'i' (a? {68 SENSOR gns SIGNAL ALARMCIRCUIT AMPLIFIER PROCESSOR Device 72 km -?o 75 Fl E-EI INVENTOR,

RAYMOND D. LUCAS AGFNT METHOD AND APPARATUS FOR OPERATING A SINGLESENSOR TO HAVE THE PROPERTIES OF AN ARRAY OF SENSORS BACKGROUND OFINVENTION This invention relates to intrusion detection systems and moreparticularly to such a system wherein a single sensor is operated tohave the properties of an array of sensors.

In reconnaissance operations it is tiring and sometimes dangerous for ahuman observer to visually monitor, even from a distance, pedestrian orvehicle traffic on a path. This function is accomplished automaticallyand electronically in a prior art system comprising an array of sensorsthat are each located on the side of a path and spaced apart so thatadjacent sensors have overlapping areas of detection on the path. Eachsensor produces an output signal when an intruder moves in theassociated detection area. A processor responsive to the sensor outputsproduces an alarm indicating that an intruder is moving on the path whenparticular outputs of a prescribed number of sensors are sequentiallyreceived within a predetermined time interval.

An object of this invention is the provision of an improved andsimplified system wherein a single sensor is operated to have theproperties of an array of sensors.

SUMMARY OF INVENTION In accordance with this invention, the false alarmrate of an intruder detection system is reduced -by decreasing the sizeof the area of detection provided by a sensor and associated controlcircuitry each time the processed output thereof indicates intrusion ofthe associated detection area and requiring indications of intrusion ofa prescribed number of detection areas of successively smaller sizewithin a predetermined time interval before initiation of an alarmindicating intrusion of a prescribed detection area. In this manner, thesensor and associated control circuitry may be adjusted to indicateintrusion of the particular detection area only when the pattern ofsensor signal activity duplicates that associated with an intruderapproaching the sensor.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic block diagram of anintrusion detection system embodying this invention;

FIG. 2 is a schematic diagram illustrating the successively smallerareas of detection associated with the system of FIG. 1;

FIG. 3 is a schematic block diagram of an alternate embodiment of thisinvention where the sensor and processing circuitry are at differentlocations:

FIG. 4 is a schematic block diagram of a radio frequency (RF) intrusiondetection system embodying this invention;

FIG. 5 is a schematic block diagram of an alternate embodiment of thisinvention in which the size of the detection area provided by the sensoris manually controlled by a human operator; and

FIG. 6 is a block diagram of an alternate embodiment of this inventionwherein detection areas'of different sizes are establishedsimultaneously.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIG. 1, anintrusion detection system embodying this invention comprises a sensorcircuit 3, processing circuit 5, transmitter 7, antennas 8 and 10, andreceiver 1 1. Circuit 3 preferably comprises a point source sensorhaving a substantially circular area of detection. The sensor may, byway of example, be: a seismic detector such as a geophone; an acousticsensor; a magnetic sensor of either the solenoid or flux gate variety;an electrostatic sensor which senses changes in the vertical componentof the earth's electrostatic field; an ultrasonic sensor; anomni-directional radar without range gates; an olfactronic sensor; anomni-directional infrared (IR) sensor; etc.. The sensor circuit may beadjusted to provide a detection area that is other than circular. Ageophone sensor 3 is placed in contact with the earth a short distance dfrom a path 15 as illustrated in FIG. 2. The spacing d is preferablymuch less than the maximum detection range associated with geophone 3.This system is particularly useful in detecting intruders 16 walking onpath 15 past geophone 3.

Processing circuit 5 comprises control circuit 17, signal processor 19,AND gate 23, and alarm circuit 24. The control circuit preferablycomprises a variable gain amplifier. Alternatively, circuit 17 maycomprise an amplifier and a variable attenuator. The gain of amplifierl7 essentially determines the sensitivity of the system and thus thesize of the detection area provided by the geophone 3. During quiescentoperation prior to detection of an intruder by the system, amplifier 17is biased by the signals on lines 25 and 26 to have a maximum gain sothat the maximum detection area corresponding to the area within curve27 in FIG. 2 is associated with the geophone. In operation, amplifier 17is sequentially responsive to control signals on lines 25 and 26 forautomatically decreasing the gain thereof and thus the sizes of thedetection areas to those within the curves 28 and 29, respectively.

Processor 19 analyzes the output of circuit 17 for producing on line 20a signal indicating whether the amplified signal from geophone 3 wasproduced by an intruder walking on the path 15 within the detectionarea. The processor may be any one of the many types of signalprocessors used to discriminate between intruder and noise generatedsignals. By way of example,

. received signals. When it is determined that an intruder is present inthe detection area, the processor produces an output pulse 30 that lastsfor a short time interval such as nano-seconds. The processor is thenprevented from producing another output pulse indicating intrusion ofthe detection area for a longer time interval such as one second eventhough the intruder remains in the detection area.

The output of the processor is applied on line 20 to gate 23, on line 21to timer circuit 31, and on line 22 to gate 32 which is enabled duringquiescent operation by the output of inverter 33. The output of AND gate32 controls the operation of a binary counter comprising circuits 36.and 37 which may be bistable multivibrators or flip-flops. The Q and 6outputs of flip-flops 36 and 37, respectively, control the operation ofAND gate 38. The Q and Q outputs of flip-flops 36 and 37, respectively,control the operation of AND gate 39. The outputs of gates 38 and 39 arethe control signals which are applied on lines 25 and 26, respectively,to amplifier 17. The output of gate 39 is also applied on line 41 to theinverter and on line 42 to gate 23.

Timer circuit 31 is set by the first output pulse 30 from processor 19.If a second pulse 30' is not produced by the processor within a firsttime interval, circuit 31 produces an output on lines 43 and 44 whichresets the associated flip-flops 36 and 37. Similarly, if a third pulse30" is not received within a second time interval, circuit 31 producesan output which resets the flip-flops. The flip-flops may also be resetby actuating spring loaded push button switch 45.

In the following operational description, the binary logic output levelsof the components in FIG. 1 will be referred to as high and low. Duringquiescent operation when an intruder is outside of the detection area27, the output of processor 19 is low; the Q and fioutputs of theflip-flops are low and high, respectively; the outputs of enabled gates38 and 39 are low; gate 32 is enabled by the high output of theinverter; and gate 23 is disabled.

When an intruder 16 walks into the detection area bounded by curve 27,the seismic signals he produces in the ground are picked up by thegeophone 3, arnplified and processed. The high output pulse 30 from theprocessor, indicating that an intruder is moving in the detection area27, enables gate 23 and closes gate 32. The low output on line 42maintains gate 23 closed. The high output of gate 32 causesmultivibrator 36 to change operating states. Since the Q and 6outputs offlip flops 36 and 37, respectively, are now both high, the output ofgate 38 is also high. This signal on line 25 causes the gain ofamplifier 17 to decrease to reduce the size of the detection area tothat bounded by the curve 28. The output of gate 39, however, is stilllow.

When the intruder 16 walks into the detection area 28, the second highoutput pulse 30' from processor 19 momentarily enables gate 23 andcloses gate 32 to cause flip-flop 36 to again change operating states.Multivibrator 37 is responsive to the leading edge of the 6 output offlip-flop 36 which is now high for changing operating states. Since bothof the inputs to gate 39 are now high, the output thereof is also high.This output of gate 39 enables gate 23, disables gate 32, and causes thegain of amplifier 17 to again decrease to reduce the size of thedetection area to that bounded by the curve 29.

When the intruder 16 walks into the smallest detection area 29, a highoutput pulse 30" is produced by the processor. This signal on line 20closes gate 23 to actuate alarm circuit 24 which energizes thetransmitter 7. Circuit 7 transmits an alarm signal to a central monitorfacility 12 indicating that successive intrusion of the detection areas27 29 have been detected and that there is an intrusion of a protectedarea adjacent geophone 3. in a self-contained unit the alarm device 24may be a light or audio device for notifying a human observer of anintrusion of the protected area.

The processing circuit may also be adjusted to operate such that thesensitivity of the system, i.e., the

size of the detection area, is increased when an intruder moves out ofexisting detection area. Such a system would be useful in monitoring anintruder who moves away from the geophone 3, e.g., on the path 15 inFIG. 2.

A modified form of this invention wherein signals produced by the sensorare transmitted to equipment in a central monitoring facility forprocessing is illustrated in FIG. 3. Circuitry in the protected areacomprises sensor circuit 3, preamplifier 51, transmitter 52 and antenna53. An antenna 55, receiver 56 and processing circuit 5' are located atthe central monitoring facility. The amplifier output of the sensorcircuit modulates a radio frequency carrier signal that is transmittedby antenna 53. The signal received by antenna is demodulated in thereceiver and applied to the processing circuit for analysis andproducing an indication of intrusion of the protected area. Theprocessing circuit 5' is similar to the processing circuit 5 illustratedin FIG. 1.

An alternate embodiment of this invention wherein a sensor circuit 3comprises a radio frequency oscillator 60 loaded by an associatedantenna 61 is illustrated in FIG. 4. The output of oscillator 60 iscombined with a local oscillator signal from source 62 in a mixer 63 toproduce an intermediate frequency (lF) signal that is amplified bycircuit 64. The amplified signal is detected by limiter-discriminator 65and applied to processing circuit 5' for analysis to determine whetheran intruder is in the protected area. The processor circuit 5" issimilar to the circuit 5 in FIG. 1. The output voltage of thediscriminator is proportional to the frequency of oscillation of RFoscillator 60. The closer an intruder is to the antenna 61, the largerthe change in the frequency of oscillation of oscillator 60 and theoutput voltage of discriminator 65. In this system, the control circuit17 and processor 19 may comprise a multi-level threshold circuit. Thesize of the detection area is decreased by increasing the magnitude ofthe threshold level of circuit 17.

When an intruder is absent from the protected area associated withantenna 61 the frequency of oscillation of oscillator 60 is constant.The output voltage of discriminator 65 is therefore a constant valuewhich is less than the threshold levels in circuit 5" and intrusion ofthe protected area is not indicated. When an intruder is in thedetection area associated with the system, however, he loads the antennaand thus oscillator 60. This causes the frequency of oscillation ofcircuit 60 to decrease. The magnitude of the frequency pulling ofoscillator 60 increases as the intruder approaches antenna 61. Thechange in the output voltage of the discriminator is detected andanalyzed by circuit 5" for varying the size of the detection area andindicating intrusion of a protected area.

In certain applications it may be desirable that a human operatorlocated in a protected area be able to selectively manually control thesystem sensitivity (i.e., the size of the detection area) fordetermining whether an intruder is approaching or leaving a particularrestricted area. A system for accomplishing this function is illustratedin FIG. 5 and comprises a sensor circuit 3, variable gain amplifier 17,processor 67 and alarm device 68. The gain of the amplifier is changedmanually by actuating spring loaded push button switches 70 72.Actuation of switch 70 causes the amplifier to have a maximum gain suchthat the system has a maximum sensitivity and the largest detection areadefined by the curve 27 in FIG. 2. The gain of the amplifier isdecreased for providing the detection areas within curves 28 and 29 byactuating switches 71 and 72, respectively. The spring loaded switch 75enables an operator to manually override an alarm indication produced bydevice 68 when changing the size of the detection area.

During quiescent operation prior to detection of an intruder, thesensitivity of the system in FIG. 5 is adjusted to be maximum. When anintruder is first detected in the area 27 and an alarm such as a lightis generated, the operator may actuate switch 75 to reset alarm device68 and suppress the alarm indication for a period of time. If intrusionof the detection area is again indicated when switch 75 is released, theoperator may selectively actuate switches 70 73 and 75 to suppress thealarm indication and vary the system sensitivity and size of thedetection area for determining whether there is actually an intrusion ofa protected area and the location thereof.

Alternatively, an operator may selectively place sensors havingoverlapping areas of detection about a central point where he is locatedwith the associated amplifiers, processors and alarm devices forselectively varying the sensitivities thereof for more accuratelydetecting an intrusion of a protected area and determining the locationof the intruder.

A system wherein a single sensor circuit 3 is employed to simultaneouslyestablish the detection areas 27, 28 and 29 is illustrated in FIG. 6.Associated control circuits A and processors B are employed with thesensor circuit 3 for establishing and determining intrusion of thedetection areas. Outputs of a prescribed number of processors B arerequired to close AND gate 77 for actuating alarm device 78 to indicateintrusion of the protected area.

What is claimed is:

l. Intrusion detection apparatus comprising means comprising a singlesensor and circuit means for selectively establishing during a suspectedintrusion into an area of detection a plurality of areas of detection ofdif ferent and predetermined sizes, and

means responsive to said sensor and circuit means for determiningintrusion of an area of detection.

2. Intrusion detection apparatus comprising means comprising a singlesensor and circuit means for selectively simultaneously establishing aplurality of areas of detection of different and predetermined sizes,and

means responsive to said sensor and said circuit means for determiningintrusion of an area of detection.

3. Intrusion detection apparatus comprising means for selectively andautomatically sequentially establishing a plurality of areas ofdetection of different and predetermined sizes comprising a singlesensor,

a control circuit responsive to the output of said sensor, said controlcircuit having different operating states associated therewith forcausing detection areas of different sizes to be associated with saidsensor,

said determining means being responsive to the output of said controlcircuit for producing an output signal indicating intrusion of anassociated detection area,

S first logic means sequentially responsive to each one of N outputs,where N is a positive interger greater than zero, of said determiningmeans indicating intrusions of the associated detection areas forautomatically biasing said control circuit to change the operating statethereof and thus the size of the detection area, and responsive to theN+l" output of said determining means for producing an alarm signalindicating intrusion of a protected area associated with the detectionareas, and

means for determining intrusion of an area of detection.

4. Apparatus according to claim 3 wherein said establishing meanscomprises a timing circuit responsive to the output of said determiningmeans for resetting said first logic means and thus said control circuitwhen an output of said determining means indicating intrusion of adetection area is not produced during a predetermined time interval. V

5. Apparatus according to claim 3 wherein said sensor circuit comprisesa radio frequency oscillator and an antenna loading said oscillator.

6. Apparatus according to claim 3 including means for coupling theoutput of said sensor to said control circuit, said coupling meanscomprising first means for transmitting signals from said sensor,

and

first means for receiving signals from said first transmitting means,

said sensor and said first transmitting means being located at adetection area,

said first receiving means, control circuit, determining means and firstlogic means being located at a monitor facility remote from said sensorcircuit.

7. Apparatus according to claim 3 including second means responsive tothe output of said first logic means for transmitting said alarm signal,and second means receiving signals from said second transmitting means,

said establishing means, determining means and second transmitting meansbeing located at a detection area,

said second receiving means being located at a monitor facility remotefrom said sensor circuit.

8. Apparatus according to claim 3 wherein said first logic meanscomprises a binary counter circuit receiving the output signal of saiddetermining means,

second logic means responsive to the contents of said counter circuitfor automatically biasing said control circuit to vary the operationthereof to sequentially change the sizes of the detection areas inresponse to successive output signals of said determining meansindicating intrusions of detection areas, and

a first logic circuit simultaneously responsive to the output of saiddetermining means and an output of said second logic means for producingan alarm signal.

9. Apparatus according to claim 8 wherein the sizes of the sequentiallyestablished detection areas are progressively smaller.

10. The method of detecting an intrusion of a protected area comprisingthe steps of establishing with a single sensor a first area of detectionassociated with the protected area,

establishing with said sensor a second area of detection associated withthe protected area, at least part of said first and second detectionareas overlapping,

detecting intrusion of said first area,

detecting intrusion of said second area, and

producing an alarm indicating intrusion of the protected area afterintrusions of the first and second areas are detected. 11. The methodaccording to claim 10 wherein said areas are established simultaneously.

12. The method according to claim 10 wherein said second detection areais established only after detecting intrusion of said first area.

13. Intrusion detection apparatus comprising means comprising a singlesensor for selectively and simultaneously establishing a plurality ofareas of detection of different and predetermined sizes,

said establishing means comprising a plurality of control circuits eachresponsive to the output of said sensor and each causing said sensor tohave a detection area of a difierent size, and

means for determining intrusion of an area of detection.

14. Apparatus according to claim 13 wherein said determining meanscomprises a plurality of signal processor circuits each responsive tothe output of an associated control circuit for producing anoutputindicating whether there is an intrusion of the associateddetection area, and

logic means responsive to the outputs of said processors for producingan indication of intrusion of a protected area associated with saiddetection areas when the outputs of a prescribed number of saidprocessors indicate intrusion of the associated detection areas.

15. Intrusion detection apparatus comprising means comprising a singlesensor and circuit means for selectively and automatically sequentiallyestablishing during a suspected intrusion into an area of detection aplurality of areas of detection of different and predetermined sizes,and

means for determining intrusion of an area of detection. 16. The methodof detecting an intrusion of a protected area comprising the steps ofestablishing with a single sensor a first area of detection associatedwith the protected area,

establishing with said sensor a second area of detection associated withthe protected area, at least part of said first and second detectionareas overpp g detecting intrusion of said first area,

detecting intrusion of said second area, and

producing an alarm indicating intrusion of the protected area only aftersimultaneous detection of intrusions of the first and second areas.

17. The method of detecting an intrusion of a protected area comprisingthe steps of establishing with a single sensor a first area of detectionassociated with the protected area,

detecting intrusion of said first area, establishing with said sensorafter detecting intrusion of said first area a second area of detectionsmaller than and located within said first detection area, detectingintrusion of said second area, and producing an alarm indicatingintrusion of the protected area after intrusions of the first and secondareas are detected.

* I I II!

1. Intrusion detection apparatus comprising means comprising a singlesensor and circuit means for selectively establishing during a suspectedintrusion into an area of detection a plurality of areas of detection ofdifferent and predetermined sizes, and means responsive to said sensorand circuit means for determining intrusion of an area of detection. 2.Intrusion detection apparatus comprising means comprising a singlesensor and circuit means for selectively simultaneously establishing aplurality of areas of detection of different and predetermined sizes,and means responsive to said sensor and said circuit means fordetermining intrusion of an area of detection.
 3. Intrusion detectionapparatus comprising means for selectively and automaticallysequentially establishing a plurality of areas of detection of differentand predetermined sizes comprising a single sensor, a control circuitresponsive to the output of said sensor, said control circuit havingdifferent operating states associated therewith for causing detectionareas of different sizes to be associated with said sensor, saiddetermining means being responsive to the output of said control circuitfor producing an output signal indicating intrusion of an associateddetection area, first logic means sequentially responsive to each one ofN outputs, where N is a positive interger greater than zero, of saiddetermining means indicating intrusions of the associated detectionareas for automatically biasing said control circuit to change theoperating state thereof and thus the size of the detection area, andresponsive to the N+1th output of said determining means for producingan alarm signal indicating intrusion of a protected area associated withthe detection areas, and means for determining intrusion of an area ofdetection.
 4. Apparatus according to claim 3 wherein said establishingmeans comprises a timing circuit responsive to the output of saiddetermining means for resetting said first logic means and thus saidcontrol circuit when an output of said determining means indicatingintrusion of a detection area is not produced during a predeterminedtime interval.
 5. Apparatus according to claim 3 wherein said sensorcircuit comprises a radio frequency oscillator and an antenna loadingsaid oscillator.
 6. Apparatus according to claim 3 including means forcoupling the output of said sensor to said control circuit, saidcoupling means comprising first means for transmitting signals from saidsensor, and first means for receiving signals from said firsttransmitting means, said sensor and said first transmitting means beinglocated at a detection area, said first receiving means, controlcircuit, determining means and first logic means being located at amonitor facility remote from said sensor circuit.
 7. Apparatus accordingto claim 3 including second means responsive to the output of said firstlogic means for transmitting said alarm signal, and second meansreceiving signals from said second transmitting means, said establishingmeans, determining means and second transmitting means being located ata detection area, said second receiving means being located at a monitorfacility remote from said sensor circuit.
 8. Apparatus according toclaim 3 wherein said first logic means comprises A binary countercircuit receiving the output signal of said determining means, secondlogic means responsive to the contents of said counter circuit forautomatically biasing said control circuit to vary the operation thereofto sequentially change the sizes of the detection areas in response tosuccessive output signals of said determining means indicatingintrusions of detection areas, and a first logic circuit simultaneouslyresponsive to the output of said determining means and an output of saidsecond logic means for producing an alarm signal.
 9. Apparatus accordingto claim 8 wherein the sizes of the sequentially established detectionareas are progressively smaller.
 10. The method of detecting anintrusion of a protected area comprising the steps of establishing witha single sensor a first area of detection associated with the protectedarea, establishing with said sensor a second area of detectionassociated with the protected area, at least part of said first andsecond detection areas overlapping, detecting intrusion of said firstarea, detecting intrusion of said second area, and producing an alarmindicating intrusion of the protected area after intrusions of the firstand second areas are detected.
 11. The method according to claim 10wherein said areas are established simultaneously.
 12. The methodaccording to claim 10 wherein said second detection area is establishedonly after detecting intrusion of said first area.
 13. Intrusiondetection apparatus comprising means comprising a single sensor forselectively and simultaneously establishing a plurality of areas ofdetection of different and predetermined sizes, said establishing meanscomprising a plurality of control circuits each responsive to the outputof said sensor and each causing said sensor to have a detection area ofa different size, and means for determining intrusion of an area ofdetection.
 14. Apparatus according to claim 13 wherein said determiningmeans comprises a plurality of signal processor circuits each responsiveto the output of an associated control circuit for producing an outputindicating whether there is an intrusion of the associated detectionarea, and logic means responsive to the outputs of said processors forproducing an indication of intrusion of a protected area associated withsaid detection areas when the outputs of a prescribed number of saidprocessors indicate intrusion of the associated detection areas. 15.Intrusion detection apparatus comprising means comprising a singlesensor and circuit means for selectively and automatically sequentiallyestablishing during a suspected intrusion into an area of detection aplurality of areas of detection of different and predetermined sizes,and means for determining intrusion of an area of detection.
 16. Themethod of detecting an intrusion of a protected area comprising thesteps of establishing with a single sensor a first area of detectionassociated with the protected area, establishing with said sensor asecond area of detection associated with the protected area, at leastpart of said first and second detection areas overlapping, detectingintrusion of said first area, detecting intrusion of said second area,and producing an alarm indicating intrusion of the protected area onlyafter simultaneous detection of intrusions of the first and secondareas.
 17. The method of detecting an intrusion of a protected areacomprising the steps of establishing with a single sensor a first areaof detection associated with the protected area, detecting intrusion ofsaid first area, establishing with said sensor after detecting intrusionof said first area a second area of detection smaller than and locatedwithin said first detection area, detecting intrusion of said secondarea, and producing an alarm indicating intrusion of the protected areaafter intrusions of the first and second areas are detected.